Why AO Smith Water Heater Won’t Light Or Stay Lit [Solved]

The main reasons why your AO Smith water heater won’t light or stay lit are:

  • Gas supply issues
  • Pilot light issues
  • Defective igniter
  • A bad thermopile 
  • A faulty gas control thermostat
AO smith water heater won't light or stay lit

However, there are plenty of other issues that can keep the pilot from staying lit. Don’t worry, we’ll discuss all of them in this guide. So, let’s dive in!

Table of Contents

AO Smith Water Heater Won’t Light Or Stay Lit [Solved]

In this section, we will break down every reason why your AO Smith water heater won’t light along with their fixes.

Note: You can also read how to fix the AO smith water heater won’t stay lit.

1. Gas Supply Issues

There are many potential issues with the gas supply that can result in the heater not lighting up. First things first, make sure that the main gas valve is not in the Off position.

When the valve is parallel to the gas lines, it means it’s in the On position. The gas valve is in the Off position when it’s perpendicular to the gas lines.

Without sufficient gas supply, your pilot light won’t light or stay light. You won’t be able to operate the heater in that condition. The other issues you need to be on the lookout for are low gas pressure and obstruction in the gas lines.


If the main gas valve was in the off position, switch it to the ON position to allow the gas supply to the unit. Then, you have to try to relight the pilot light. Follow these steps to do it:

  • Turn the gas supply off to your heater and leave it for at least 10 minutes to cool down before attempting to relight it. The control knob of the thermostat should be in the Off position.
  • Place the gas control knob in the pilot position. Press the knob and hold it down.
  • Keep clicking the igniter once every second while holding the gas control knob and do it for 90 seconds.
  • Release the gas control knob when the status light starts blinking. Then, you can adjust the control knob to your desired temperature settings and enjoy the hot water.

If the status light doesn’t blink after clicking the igniter, you can try to relight the pilot light again. Do it 3-5 times while waiting for 10 minutes during each time so that the gas is cleared.

The pilot light should come on after these attempts if there are no other issues with your heater. If your pilot light still won’t light or stay light, keep reading as we discuss more issues that can cause it.

Next, you have to make sure that there are no obstructions in the gas lines. If your gas lines are old and you suspect that dirt has gotten into them, you should replace them.

There can also be issues with the gas pressure. In that case, you need to check it with a manometer. The minimum gas pressure for AO Smith heaters when using natural gas is 5.0” W.C. If the pressure is lower than that, contact your local gas provider to fix the issue.

If you’re using LP gas, make sure you haven’t run out of gas. You can just refill the tank or replace it with a new one to solve the issue. The flex tube can also get kinked and interrupt the proper flow of gas. So, make sure you straighten it out or replace it so that the pilot stays lit.

2. Pilot Light Issues

Lighting the pilot is the first step to making sure that your heater operates properly. A pilot light ignites the main burner when there is a demand for hot water.


A faulty pilot is the next major factor behind the AO Smith gas water heater not lighting. Sometimes, there may be a buildup of carbon deposits that restricts the operation of the pilot light.

The component that lights the pilot is called a spark electrode. As the name suggests, it creates sparks so that the pilot lights up. When something goes wrong with it, the pilot won’t light either.

Basically, the spark electrode allows an intermittent high voltage to light the pilot flame. You won’t see any spark there if the electrode or anode rod gets faulty or worn out.

Plus, over time, the pilot light electrode tip may be obstructed by carbon deposits, dirt, and rust that restrict the igniter from releasing the sparks. So, the pilot light won’t ignite because of this lack of sparks.

Finally, a blocked pilot tube can also be the main culprit why the pilot won’t light or stay light. When the pilot tube is blocked by dirt, dust, and debris, that restricts the flow of gas. And without proper fuel getting into the pilot light, it’s not going to produce a strong flame.


First, try to relight the pilot. If it doesn’t light up normally, here’s a trick that can work for you:

  • After you attempt to relight the pilot light and the status light blinks four times instead of working properly, you have to reset the AO smith water heater.
  • Turn the gas control knob to the “Very Hot” position for 10 seconds. Then, move it to “Hot” and then, move it to the “Pilot” position for 10 seconds each.
  • The status light should go back to normal if you do this two to three times. Then, move the knob to the “Hot” position again and the pilot light should come on.

If the pilot still doesn’t light, check the pilot tube. If there are any restrictions or debris in the pilot tube, try to clear them out with a stiff brush or compressed air. 

Finally, it may be due to the faulty spark electrode. You’d have to replace it but you may not find it as a single unit. So, you’d have to replace the entire pilot light assembly to solve this issue.

3. Defective Igniter

Your AO Smith water heater pilot light won’t stay lit if the ignitor doesn’t work properly. The igniter must have sufficient voltage to light the pilot.

AO smith water heater defective igniter

The issue can also be with the igniter wires. If they are worn out or damaged in any way, they will also keep the pilot from lighting. If you don’t see any sparks when you press the igniter, that’s a strong indicator of something being wrong with the igniter itself or the wires.


Check the igniter wires first as it’d be easier to do a visual inspection on them. If they turn out to be damaged, you have to replace them. If that doesn’t work, you have to replace the igniter. Hopefully, doing so will light your heater.

4. Faulty Thermopile

Another potential reason why your AO Smith gas water heater won’t stay lit is a defective thermopile. It’ll become clear why it happens once you get to know how the thermopile works.

AO smith water heater thermopile

The thermopile detects heat on the pilot flame and sends an output voltage. It indicates that your heater is working properly and the main gas burner lights up after that.

But when the thermopile goes bad, it doesn’t detect the heat from the pilot light. So, it signals the heater to shut off the operation and that’s why the pilot light goes out.

It’s also possible that the thermopile has moved away from the pilot light and that’s why it’s not able to detect the flame. If the status light in your heater flashes twice, it means that the pilot is lit but the thermopile isn’t producing the right voltage.

Some other signs of thermocouple issues are:

  • The pilot light is on but the burner won’t stay lit.
  • The pilot is lit but it went out when you released the control knob.
  • The status light won’t come on for 90 seconds after the pilot light operation.


The first step to fix this issue is to inspect the thermopile. So, turn off the by setting the gas control knob to the off position and wait for the heater to cool down.

Then, remove the burner access door and locate the thermopile. Examine it to see if it gets clogged or bad. Pull out the thermopile with a screwdriver. Ensure the thermopile has no rust. If it gets rusty, clean it with sandpaper and scrape off the dirt so that the pilot can light up.

The thermopile can also be bent in a bad way and sit far from the pilot light. So, make sure it’s close enough to the pilot light when you put it back in. Try to relight the pilot after doing that.

If you fail to light the pilot after multiple attempts, then replace the thermopile. You can do it on your own as you were able to pull out the thermopile from cleaning it. Now, all you have to do is to replace it with the right part. 

5. Faulty Gas Control Thermostat

When either cleaning the pilot light or replacing the thermopile doesn’t help ignite your water heater, the issue might be with the defective gas control thermostat. 

faulty gas control thermostate

The gas control thermostat is an essential element that measures the amount and pressure of gas needed in the unit and allows the pilot to stay lit.  And then, the lit pilot heats the thermopile that delivers a signal to the gas valve to operate the burner in order to heat the water.

Unfortunately, if the control valve doesn’t get the signal, it cannot produce the right amount of gas. So, it won’t allow your heater to heat and stay lit. This may happen if the gas valve gets defective or broken. The status light flashes seven times to signal a bad thermostat.


The gas control thermostat is not a serviceable part. You’d have to replace it if you want your heater to stay lit. Follow these steps to replace the gas control thermostat:

Step 01 – Remove The Connections

  • Move the gas control knob to the “Off” position and turn off the main gas supply.
  • Drain the water heater and make sure there is no water in the heater.
  • Remove the connection between the igniter wire and the igniter lead wire. Then, remove the thermocouple wires as well as the pilot tube, and the manifold tube.
  • Detach the gas joint union from the gas piping.

Step 02 – Replace The Gas Control Thermostat

  • Use a 4” section of gas pipe to turn the gas control thermostat counterclockwise and remove it.
  • Install a new gas control thermostat in place of the old one and use the same 4” section of gas pipe to turn it clockwise.
  • Tighten the gas control thermostat but make sure you don’t overtighten it.

Step 03 – Test For Leaks

  • Put everything back together in reverse order and make sure there are no gas leaks. You can use soapy water to test for leaks. If there are leaks, you’d see the formation of bubbles.
  • If there are no leaks, fill the tank completely with water before turning it on.
  • If there is a gas leakage, you have to do the installation again so that there are no such leaks. If you can’t do it yourself correctly, consult a qualified technician.

That’s how you can easily replace the thermostat on a gas water heater. You’ll get hot water from your heater again and it’ll stay light if a faulty gas control thermostat was causing the issue.


What is a water heater pilot light?

A pilot light is an essential part of a water heater. It provides a flame to light, including the main burner so that the water inside the tank can be heated. Finally, your heater runs smoothly.

Does the AO Smith water heater pilot light stay lit all the time?

A gas water heater has a small flame (called a “pilot light”) to ignite the burner. The pilot light ignites the burner to heat the water. So, it is normal to stay light all the time unless you turn it off.

What might happen if the pilot doesn’t light up?

If the pilot light isn’t lit at all, the main burner won’t ignite and your heater won’t be able to keep warm water and you’ll get cold water instead. So, fix the pilot light issue ASAP.


Fortunately, we have already figured out the root reasons why your AO Smith water heater won’t light or stay lit and nicely explained how to fix them by yourself.

So, whenever you face any lighting issues with your AO water heater, this guide will definitely be effective for you to solve the issues.

Yet, if you have questions about your AO Smith water heater’s lighting issue, let us know by your comments. We will reach out to you with a possible solution ASAP.

About William

William is the founder of Fireplacehubs.com. He has real life practical skills in fixing smoker & heating appliance issues. He loves to share his knowledge & helps others in fixing their appliances & saving their money. William firmly believes that anyone can repair his or her unit with the correct guidance & knowledge. See more at about us.

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